Hooking up multiple RGB LEDs while using a minimal number of PWM pins on an Arduino?
Fairly simple. Just using npn transistors. Duplicate per color channel.
Your transistor can sink up to 600mA continuously. At 20mA per led, 4 per channel, that's only 80mA. That's more than enough.
But make sure your power supply can support that. 80 * 3, that's 240mA on the leds alone, not including the rest of the arduino, the transistor base, anything else you are using.
OP Provided a Fritzing Diagram of the answer as well:
These diagrams/images might be better to visually see whats going on:
